Flow control of gases used in the fabrication of semiconductors and similar manufacturing processes is critical to providing quality products. In semiconductor manufacturing, for example, certain gases directly influence the chemical and physical processes that deposit material on or etch material off of a semiconductor wafer. As the semiconductor industry continues to miniaturize semiconductor devices, the demand for accurate flow control of gases used in semiconductor manufacturing processes has become even more critical. Shorter manufacturing process times and smaller quantities and flow rates of gas. are being required. The demand for more accurate process gas flow control is aggravated by conditions such as fluctuating pressures in gas supply manifolds and conduits, requirements to control very low gas flow rates and the requirements to reduce or eliminate so-called xe2x80x9cburst flowsxe2x80x9d which occur when valves in the gas supply conduit flow paths are opened suddenly to release pressurized gas to the process chambers.
Accordingly, there has been a need to improve fluid mass flow control apparatus used for control of process gas in semiconductor manufacturing, in particular. In this regard, there has been a need to develop fluid mass flow control apparatus wherein the fluid volume in the apparatus is minimized in order to accurately control the flow of relatively small quantities of gas and to minimize the requirements to purge moisture and previously controlled gases from the apparatus when a change in the type of gas being controlled is undertaken. By shrinking the so-called accumulation volume within the mass flow control apparatus, false flow signals to the apparatus control system are reduced and perturbations in the fluid flow output of the apparatus are also reduced. For example, with certain prior art mass flow control apparatus, minor changes in pressures in common gas supply manifolds supplying one or more mass flow controllers can affect closure of the flow control valves of one or more of the controllers wherein the controller(s) can only regain correct flow control after an elapsed time on the order of one to four seconds. Considering that some semiconductor manufacturing processes last only a total of five seconds, such lengthy recovery times for the flow control apparatus can significantly affect the associated process.
Prior art mass flow controllers are, for example, subject to reading and reacting on so-called false flow signals. A change of fluid pressure in the so-called accumulation volume of the controller between the section of the controller which includes the flow measuring sensor and flow restrictor and the flow control valve may be caused by additional fluid mass entering or exiting the accumulation volume. A majority of such fluid mass enters or exits the accumulation volume through the flow measuring section due to the relatively low resistance to flow through that section of the controller compared to flow resistance through the flow control valve seat. Accordingly, a change in the supply pressure to the mass flow controller or a change in the flow rate through the mass flow controller will result in a change in the quantity of fluid mass being measured and flowing through the mass flow controller""s measuring section. However, a major portion of this flow does not pass through the flow control valve and out of the controller but, since the function of the mass flow controller is to indicate and control the flow out of the controller, these measured flows resulting from pressure increases or decreases are considered false or erroneous. Consequently, the control system for the mass flow controller, utilizing false flow information, responds by improperly positioning the flow control valve resulting in an improper flow through the flow control valve and out of the controller until the transient pressure change has expired. The relative size of an anticipated error in flow output can be significant when flow rates are relatively low such as, for example, about 100 SCCM (standard cubic centimeters per minute). Moreover, in some process applications, fluid mass flow control apparatus are required to operate with full scale flow ratings as low as 1.0 SCCM.
In addition to the flow control problems in mass fluid flow control apparatus associated with low fluid flow rates, so-called dry-down performance deteriorates at lower flow rate requirements. Dry-down performance refers to the time required for moisture to be purged from the surfaces of the fluid mass flow control apparatus exposed to the process gases being controlled. Of course, the moisture content and velocity of gas flowing over wetted surfaces influences the rate at which moisture is removed therefrom. Flow path geometry, cross-sectional flow area and surface area are important parameters to be considered when trying to maximize the ability to remove moisture and purge unwanted gases from the flow passages of fluid mass flow control apparatus.
As mentioned above, the reduction or elimination of so-called burst flow is increasingly being required in certain fluid mass flow control apparatus applications where, for example, an isolation valve is located in the fluid flow path downstream of the mass flow controller. Fluid mass flow control apparatus typically do not have positive full flow shutoff capability but may limit flow to a fraction of one percent of the full flow capability of the apparatus. The pulse flow of gas associated with the pressurized volume of gas within the apparatus and the opening of the isolation valve means a loss of flow control. Conversely, in arrangements where the isolation valve is disposed upstream of the mass flow controller, it is desirable, in many instances, to maintain a zero flow set point command to the mass flow control apparatus and open the isolation valve a short period of time before changing the flow command. In this way the controller will perform in a more controlled manner so that, once the transient supply pressure conditions associated with opening the isolation valve have expired, the mass flow controller operating set point can be changed.
In light of the performance problems discussed hereinabove and further hereinbelow, it becomes apparent that reducing the internal volume and surface area of fluid mass flow control apparatus is necessary to meet increasingly stringent performance requirements. It is to these ends that the present invention has been developed.
The present invention provides an improved fluid mass flow control apparatus, particularly of a type required for controlling relatively small fluid mass flow rates, including, for example, fluid mass flow rates required in production processes for semiconductor devices. The present invention also provides an improved fluid mass flow control apparatus having a small internal volume, a unique flow passage arrangement, an improved actuator for actuating a flow control valve, an improved flow control valve arrangement and an improved arrangement for adjusting the apparatus to set the full flow control range of the apparatus.
In accordance with one aspect of the invention, a fluid mass flow control apparatus is provided which includes two parallel flow passages constructed from relatively small diameter tubing sections. One of the flow passages provides for routing part of the flow through a mass flow sensor and the other of the passages is configured such as to serve as a flow restrictor. By utilizing smaller flow passages higher pressure drops therethrough at moderate to high flow rates can be tolerated without compromising performance. Moreover, the smaller diameter flow passages and a simplified flow restrictor provide improved apparatus performance while retaining low fluid pressure drops through the controller at low fluid flow rates. An adjustment may be made by inserting a wire or tube of smaller diameter into the tube which leads to and includes the flow sensor. By changing the diameter and the length of the inserted wire or tube, the flow restriction of the sensor tube can be adjusted to achieve the desired flow range.
The present invention also provides an improved fluid mass flow control apparatus which utilizes a flow control valve capsule in which a valve closure member and valve seat is assembled. The valve closure member may, if desired, be spring biased to its closed position. The so-called valve capsule is also configured to cooperate with an improved valve actuator.
Still further, the invention contemplates the provision of a valve actuator for a fluid mass flow control apparatus which is disposed in and partially defines the fluid flowpath, is capable of withstanding exposure to corrosive fluids and is disposed in a position to provide a more efficient and simplified mechanical design. A magnetostrictive actuator, in particular, may be used to move the valve closure member. A magnetostrictive actuator is preferred considering manufacturing costs, simplicity and reliability of design and resistance to damage from corrosive fluids.
In accordance with a still further aspect of the present invention, a fluid mass flow control apparatus is provided which includes small internal fluid volume to minimize control perturbations, to improve fluid purging and so-called dry-down performance and to generally improve the control of relatively small volumes of fluids, particularly gases used in semiconductor manufacturing processes.
Those skilled in the art will further appreciate the above-mentioned advantages and superior features of the invention together with other important aspects thereof upon reading the detailed description which follows in conjunction with the drawings.